Rubber compositions containing non-sulfur silica coupling agents bound to diene rubbers

ABSTRACT

The invention provides vulcanizable rubber compound having improved tensile mechanical and dynamic viscoelastic properties. The compounds are formed by mixing an elastomer, containing unsaturated carbon-carbon bonds in its molecular structure, with a reinforcing inorganic filler silica in the presence of an alkyl alkoxysilane and a non-sulfur coupling agent that binds to the rubber backbone with an “ene” linkage or a 1,3 dipolar addition linkage. In particular, the coupling agent and the alkyl alkoxysilane are present in the compound in a weight ratio of about 0.0001:1 to about 1:1.

This Application claims priority to U.S. Provisional Application Ser.No. 61/118,781 filed Dec. 1, 2008.

BACKGROUND OF THE INVENTION

When producing elastomeric compositions for use in rubber articles, suchas tires, power belts, and the like, it is desirable that theseelastomeric compositions are easily processable during compounding andhave a high molecular weight with a controlled molecular weightdistribution, glass transition temperature (T_(g)) and vinyl content. Itis also desirable that reinforcing fillers, such as silica and/or carbonblack, be well dispersed throughout the rubber in order to improvevarious physical properties, such as the compound Mooney viscosity,modulus, tangent delta (tan δ), and the like. Rubber articles,especially tires, produced from vulcanized elastomers exhibiting theseimproved properties will have reduced hysteresis, better rollingresistance, snow and ice traction, wet traction, and improved fueleconomy for vehicles equipped with such tires.

Dispersion of silica filler has been a major concern in tire processingbecause polar silanol groups on the surface of silica particles tend toself-associate and reagglomeration of silica particles can occur aftercompounding, leading to poor silica dispersion and a high compoundviscosity. The strong silica filler network results in a rigid uncuredcompound that is difficult to process in extrusion and formingoperations. Various silica coupling agents and silica shielding agentshave been employed to address these problems. For example, bifunctionalsilica coupling agents having a moiety (e.g., a silyl group) reactivewith the silica surface and a moiety (e.g., sulfur) that binds to theelastomer are well known and include mercaptosilanes, blockedmercaptosilanes, bis(trialkoxysilylorgano) polysulfides, and the like.These commonly-used sulfur-containing bifunctional silica couplingagents can offer excellent coupling between rubber and silica; however,there are disadvantages to their use. For example, the high chemicalreactivity of the —SH functions of the mercaptosilanes (e.g.,γ-mercaptoalkyltrialkoxysilanes, 3-thiocyanatopropyl trimethoxysilane,and the like) with organic polymers can lead to unacceptably highviscosities during processing and to premature curing (scorch), makingcompounding and processing more difficult. Rubber compounds employingbis(trialkoxysilylorgano) polysulfides often have mixing temperaturelimitations to avoid thermal degradation of the agents and prematureincrease in the viscosity of the rubber mixture. In general, whencompared with carbon black-filled compositions, a silica coupling agentis needed to obtain tread compounds having good silica dispersion.

Several approaches to improving dispersion of silica in rubber compoundshave been directed to reducing or replacing the use of suchsulfur-containing silica coupling agents by employing silica dispersingaids, such as silica shielding agents that chemically react with, and/orphysically shield the surface silanol groups on the silica particles butare not reactive with the elastomer. Although these agents can improveprocessability of the compound, in some cases they may not provideadequate physical properties to the rubber compound when used alonebecause they do not chemically bind to the rubber backbone.

Recently, non-sulfur alkoxysilane coupling agents, such ascitraconimido-alkoxysilane coupling agents, have been described in U.S.Pat. Nos. 6,878,768 and 7,238,740. However, a major problem with usingthese silica/rubber coupling agents is that they are only effective whencombined with an isoprene elastomer, and are ineffective in the absenceof such an elastomer. That is, when the diene elastomer consistsessentially of another synthetic elastomer such as styrene-butadienerubber (SBR), conventionally used in treads for tires, the excessivereactivity, increase in viscosity and insufficient coupling performance,are still a major problem. Moreover, it was disclosed that, despite asimilar chemistry, a similar maleimido-triethoxysilane coupling agentproduced a greatly increased viscosity in the uncured isoprene rubbercompared with the citraconimido-trialkoxysilane coupling agent.

Therefore, there is a need for rubber compositions that do not have theabove-described limitations, regardless of the diene rubber employed,that can make use of non-sulfur silica coupling agents that can bond tothe rubber backbone.

SUMMARY OF THE INVENTION

In all arrangements of the invention rubber compositions, unlessotherwise indicated, the term “silica” as used herein as a reinforcingfiller, is intended to include other known inorganic reinforcing fillersfor rubber such as, but not limited to, clay (hydrous aluminumsilicate), talc (hydrous magnesium silicate), kaolin, clay, metaloxides, aluminum hydrate [Al(OH)₃], mica, and the like.

It was unexpectedly discovered that the detrimental effects of rapiddiene rubber binding by non-sulfur eneophile alkoxysilane couplingagents such as, but not limited to, maleimide alkoxysilanes, citronimidealkoxysilanes, azo-bis-carbonyl alkoxysilanes, and the like, can beovercome by using very small amounts of these silica/rubber couplingagents in combination with an additional silica shielding agent thatdoes not bind to the rubber. Moreover, it has unexpectedly beendiscovered that the detrimental effects of rapid diene rubber binding byother non-sulfur rubber binding alkoxysilane coupling agents such as,but not limited to, and alkoxysilane compounds that bind to the rubberby 1,3 dipolar addition such as nitrone alkoxysilanes, nitrile iminealkoxysilanes, nitrile oxide alkoxysilanes, and the like, similarly canbe overcome by the inclusion in the rubber composition of the additionalalkoxysilane silica shielding agent. That is, a very small amount of theeneophilic alkoxysilanes and those alkoxysilanes having groups that bindto the rubber by 1,3 dipolar addition and facilitate binding of silicaby the polymer, in combination with a much larger amount of the silicashielding agent, provides a desirable compound viscosity forprocessability and results in cured rubber having reduced hysteresis andstress/strain properties.

An advantage of the arrangements of the invention is that the rubbercompositions containing the appropriate weight ratio of thesilica/rubber coupling agent and the silica shielding agent results in avulcanizable and/or vulcanized elastomeric composition, containingvirtually any olefin rubber or combination of olefin rubbers, thatdemonstrates tensile mechanical and dynamic viscoelastic properties thatare improved over similar compounds prepared with the eneophilealkoxysilanes and/or the 1,3 dipolar addition alkoxysilane compoundsalone. Further, the properties of the invention compounds exhibitimproved dispersion of silica, reduced filler flocculation aftercompounding and increased bound rubber content, resulting in lowerhysteresis and improved wear resistance in the vulcanized product,comparable to or improved over compounds containing abis(trialkoxysilylorgano) polysulfide, and comparable properties tocompounds prepared with a similar weight ratio of a mercaptosilane andan alkyl alkoxysilane.

In particular, a vulcanizable rubber composition comprises an elastomercontaining unsaturated carbon-carbon bonds in its molecular structure; areinforcing inorganic filler or a mixture thereof with carbon black; acoupling agent comprising an alkoxysilane moiety that binds to theinorganic filler and a non-sulfur moiety that reacts with theunsaturated carbon-carbon bonds of the elastomer to bind the couplingagent to the elastomer; an alkyl alkoxysilane, wherein the weight ratioof the coupling agent to the alkyl alkoxysilane is about 0.0001:1 toabout 1:1; and a cure agent. As a non-limiting example, thesilica/rubber coupling agent in the rubber composition can comprises animidoalkoxysilane, an azo-bis-carbonyl-alkoxysilane, a nitronealkoxysilane, a nitrileimine alkoxysilane, a nitrileoxide alkoxysilane,and mixtures of these. The elastomer is selected from the groupconsisting of homopolymers of a conjugated diene monomer, and copolymersand terpolymers of the conjugated diene monomers with monovinyl aromaticmonomers and trienes and, especially copolymers and terpolymers ofbutadiene that comprise about 35% or greater butadiene content.

The invention further provides a vulcanized rubber compound comprisingthe vulcanizable rubber composition, and a pneumatic tire comprising acomponent produced from the vulcanized rubber compound.

DETAILED DESCRIPTION OF THE INVENTION

The terms elastomer, polymer and rubber are used interchangeably herein,as is customary in the rubber industry.

The invention presents a vulcanizable rubber composition that comprisesan elastomer containing unsaturated carbon-carbon bonds in its molecularstructure; a reinforcing inorganic filler, carbon black, or a mixture ofthe inorganic filler with carbon black; a coupling agent comprising analkoxysilane moiety that binds to the inorganic filler and a non-sulfurmoiety that reacts with the unsaturated carbon-carbon bonds of theelastomer to bind the coupling agent to the elastomer; an alkylalkoxysilane, wherein the weight ratio of the coupling agent to thealkyl alkoxysilane is about 0.0001:1 to about 1:1; and a cure agent.

The non-sulfur moiety of the coupling agent can include, but is notlimited to, an eneophile, a compound that forms a 1,3 dipolar additionlinkage to the rubber and, in the vulcanizable rubber composition, caninclude mixtures of these.

The coupling agent can comprise, but is not limited to, unsaturatedimidosilane compounds having the formula:Y—(R³)(R²)_((3-a))Si(OR¹)_(a),

wherein R¹ is C₁ to C₄ alkyl; R² is C₁ to C₂₀ alkyl, C₃ to C₂₀cycloalkyl, C₆ to C₂₀ aromatic or C₅ to C₂₀ heteroaromatic; R³ is C₁ toC₂₀ alkylenyl, C₃ to C₂₀ cycloalkylenyl, C₆ to C₂₀ arylenyl, C₅ to C₂₀heteroarylenyl, or R⁴—CH═CH—R⁴, where R⁴ is a single bond or an R³; a=1,2 or 3; and Y is selected from the group consisting of

-   -   (i) —N(C═O)₂N═N,    -   (ii) —N(C═O)₂CH═CR¹,    -   (iii) —NH(C═O)N═N(C═O)OR¹,    -   (iv) —(C═O)N═N(C═O)OR¹,    -   (v) —O(C═O)N═N(C═O)OR¹,    -   (vi) —C≡N⁺→(O)⁻—R²,    -   (vii) —C≡N⁺→N⁻—R²,    -   (viii) —C≡N⁺→O⁻, and mixtures thereof.

Examples of such coupling agent include, but are not limited to,imidoalkoxysilanes, azo-bis-carbonyl-alkoxysilanes, nitronealkoxysilanes, nitrileimine alkoxysilanes, nitrileoxide alkoxysilanes,and the like. Combinations of any of these can be used in the rubbercompositions according to the invention. In particular, such couplingagents can include trialkoxysilanes, dialkoxysilanes, monoalkoxysilanes,and the like.

In contrast to the restriction of using a citraconimido-alkoxysilanewith an isoprene rubber, described in U.S. Pat. No. 7,238,740, each ofthe non-sulfur coupling agents in combination with an alkyl alkoxysilanesilica shielding agent, according to the invention, can be used with anyelastomer containing unsaturated carbon-carbon bonds in its molecularstructure. For example, the following formulas illustrate chemistry forattachment of alkoxysilanes to rubber backbones using non-sulfurlinkages.

I. ENE Chemistry

In the following formulas, R is equivalent to R¹, and R′ is equivalentto R² above.

A. Maleimides

Structure 1 below is representative of a maleimide alkoxysilanesilica/rubber coupling agent, namely N-(3-propyltriethoxysilyl)maleimide prepared by the reaction of maleic anhydride and3-aminopropyl-triethoxysilane. Structure 2 below is representative of acitraconimide alkoxysilane silica/rubber coupling agent, namelyN-(3-propyltriethoxysilyl) citraconimide prepared similarly from thereaction of citraconic anhydride and 3-aminopropyltriethoxysilane. Thesynthesis of both structures is described in U.S. Pat. Nos. 6,878,768and 7,238,740 using a process for the synthesis of N-alkyl- andN-arylimide derivatives, as described in J. Organic Chemistry, Vol. 62,2652-2654, 1997.

As described in the above patents, structure 2 was an effective couplingagent that bonded to isoprene containing rubbers but was poorly reactivewith butadiene rubbers, such as SBR. In contrast, structure 1 gave anundesirable high compound ML₄ with isoprene containing rubbers. It isknown that maleimides have a high chemical reactivity with dienerubbers, i.e., they react extremely quickly with the rubber, resultingin rubber compounds that, like those using mercaptosilane couplingagents, can lead to unacceptably high viscosities during processing andto premature curing (scorch), making compounding and processing moredifficult.

Without being bound by theory, an explanation of the reported reactivitycan be understood from the mechanism of the of “ene” addition to thedouble bond of the isoprene rubber. This involves the allyl proton ofthe rubber being shifted to the eneophile in a concerted manner asillustrated in Scheme 1 below. Thus the rate of the reaction iscontrolled by the number of allyl hydrogens and the steric factor of thetransition state.

B. Azo-bis-carbonyls

Three different variations listed below as A, B and C, can be used toprepare an exemplary intermediate compound with the structure ofEtCO₂NHNHCO—Y—R-TS

where Et is an ethyl group; the —Y— group represents a single bond,oxygen or nitrogen; R is derived from an acid chloride, achlorocarbonate or an isocyanate; and TS represents a trialkoxysilane.As a non-limiting example, these attachments can be introduced to theazo-bis-carbonyl compound, ethyl carbazate, by reaction with atrialkoxysilane (such as those available from Gelest, Morrisville, Pa.)in, at most, one additional reaction step:

A) a phosgene reaction withN-(3-triethoxysilylpropyl)-4-hydroxybutyramide;

B) a thionyl chloride reaction with triethoxysilylpropyl maleamic acid;or

C) using 3-isocyantopropyl trimethoxysilane.

For example, some ethyl carbazate reactions with trialkoxysilanes areillustrated below in Examples 2A, 2B and 2C. However, the illustratedreactions are not limited, as other chemical reactions to attachalkoxysilanes to coupling agents, such as eneophiles and/or 1,3 dipolaraddition coupling agents are known to those of ordinary skill in theart.

The intermediate product thus obtained can then be oxidized with N₂O₄ togive the “ene” reactive compounds where Y would be the desired singlebond, oxygen or nitrogen suggested above. The reaction with any olefinrubber will give the following.

II. 1,3 Dipolar Addition

The use of a 1,3-dipolar addition reaction with the unsaturated rubbergives a good addition site for any trialkoxysilane containing moleculeto which it is attached. The following represents the type of structurethat can be used.

A. Nitrones

Nitrones are good dipolar addition reagents which have been usedrecently to attach a carbon black reactive functionality to anunsaturated rubber, as described in U.S. Pat. No. 7,186,845. They can beprepared with an aldehyde functionalized trialkoxysilane, such astriethoxysilyl butyraldehyde or triethoxysilyl undecanal, by a reactionwith phenyl hydroxylamine, as illustrated in Scheme 2 below.

The reaction with rubber gives the structure of the attachment shownbelow.

A trialkoxysilane can be introduced, for example, by employing an ethylcarbazate reaction with a trialkoxysilane, as described above and alsoin examples 2A, 2B and 2C below.

B. Nitrile Imines

Another useful unsaturated rubber reactive compound has atriethoxysilane attached to a nitrile imine. The nitrile imine can beformed by a process illustrated in Scheme 3.

The reaction with rubber gives the structure of the attachment shownbelow.

The intermediate acid chlorides from IIB, i.e., a thionyl chloridereaction with triethoxysilylpropyl maleamic acid, can be reacted withmethyl hydrazine, followed by chlorination and dehydrohalogenation witha base, to give the nitrile imine. As discussed above, the reactivity ofthe product can be modified by the type of R′ used and the coupling canbe moderated as mentioned in IA above, and the trialkoxysilane can beintroduced by the reaction of, for example, ethyl carbazate with atrialkoxysilane, as described above.

C. Nitrile Oxide

This type of dipolar compound can readily be synthesized from aldehydesin the following manner, similar to the nitrones depicted above.However, this scheme requires additional steps of chlorination anddehydrohalogenation to obtain the desired triethoxysilane product.

The reaction of this compound with unsaturated rubber gives thefollowing product.

Trialkoxysilane group(s) can be added, as described above and in example3, below.

In contrast to the teachings of U.S. Pat. Nos. 7,238,740 and 6,878,768,and according to arrangements of the present invention, it wasunexpectedly discovered that the detrimental effects of rapid dienerubber binding by eneophiles such as, but not limited to, maleimidealkoxysilanes, citronimide alkoxysilanes and azo-bis-carbonylalkoxysilanes, and alkoxysilane compounds that bind to the rubber by 1,3dipolar addition, can be overcome by using very small amounts of thesesilica/rubber coupling agents in combination with an additional silicashielding agent that shields other silica present in the composition butdoes not bind to the rubber. That is, the eneophilic alkoxysilanes andthose alkoxysilanes having groups that bind to the rubber by 1,3 dipolaraddition, facilitate binding of silica by the polymer, and the silicashielding agent provides a desirable compound viscosity forprocessability. Thus the appropriate combination of the silica/rubbercoupling agent and the silica shielding agent results in a vulcanizableand/or vulcanized elastomeric composition, containing virtually anyolefin rubber or combination of olefin rubbers, that demonstratestensile mechanical and dynamic viscoelastic properties that are improvedover similar compounds prepared with either the maleimide alkoxysilaneor the citraconimide alkoxysilane alone (isoprene excepted). Further,the properties of the invention compounds exhibit improved dispersion ofsilica, reduced filler flocculation after compounding and increasedbound rubber content, resulting in lower hysteresis and improved wearresistance in the vulcanized product, comparable to or improved overcompounds containing a bis(trialkoxysilylorgano) polysulfide.Silica-reinforced rubber stocks containing small amounts ofsulfur-containing mercaptosilane coupling agents and larger amounts ofalkyl alkoxysilanes are described in our U.S. Pat. No. 6,433,065.However, it was unexpected to discover that the present stockscontaining a combination of small amounts of non sulfur-containingsilica coupling agents and large amounts of alkyl alkoxysilanes alsoprovided desirable rubber properties.

The weight ratio of the coupling agent to the alkyl alkoxysilane can beabout 0.0001:1 to about 1:1, especially about 0.001:1 to about 0.80:1,also about 0.002:1 to about 0.60:1 and about 0.005:1 to about 0.40:1, orabout 0.01:1 to about 0.20:1. The coupling agent is present in an amountof about 0.0001% to about 3% by weight, based on the weight of thesilica, or about 0.001% to about 1.5% by weight or about 0.01% to about1% by weight, based on the weight of the silica. The alkyl alkoxysilanecan also be present in an amount of about 0.1% to about 20% by weight,based on the weight of the silica. Usually, the alkyl alkoxysilane ispresent in an amount of about 1% to about 15% by weight and, often, inan amount of about 1% to about 10% by weight, based on the weight of thesilica.

Alkoxysilanes suitable for use in the invention compounds have theformula R¹ _(p)Si(OR²)_(4-p), wherein the alkoxy groups are the same ordifferent from each other, each R¹ independently comprises C₁, to aboutC₂₀ aliphatic, about C₅ to about C₂₀ cycloaliphatic, or about C₆ toabout C₂₀ aromatic, each R² independently comprises C₁ to about C₆aliphatic, and p is an integer from 1 to 3. In another arrangement, atleast one R¹ contains from 6 to 20 carbon atoms and the remainder of theR¹ groups, if any, contain from 1 to 3 carbon atoms. The R² can contain1 to 4, more preferably 1 or 2, carbon atoms. In some arrangements, R²is an alkyl group. Suitably, at least one R¹ is much larger in terms ofcarbon atoms than an R² contained in the alkoxy groups of the silane.

Typically, the alkyl alkoxysilane is a trialkoxysilane such as, but notlimited to, octyltriethoxysilane, octyltrimethoxysilane,trimethylethoxysilane, cyclohexyltriethoxysilane,isobutyltriethoxysilane, ethyltrimethoxysilane,cyclohexyltributoxysilane, dimethyldiethoxysilane,methyltriethoxysilane, propyltriethoxysilane, hexyltriethoxysilane,heptyltriethoxysilane, nonyltriethoxysilane, decyltriethoxysilane,dodecyltriethoxysilane, tetradecyltriethoxysilane,octadecyltriethoxysilane, methyloctyldiethoxysilane,dimethyl-dimethoxysilane, methyltrimethoxysilane,propyltrimethoxysilane, hexyltrimethoxysilane, heptyltrimethoxysilane,nonyltrimethoxysilane, decyltrimethoxysilane, dodecyltrimethoxysilane,tetradecyltrimethoxysilane, octadecyltrimethoxysilane,methyloctyldimethoxysilane, mixtures of these, and the like. Often thealkyl alkoxysilane is selected from at least one ofn-octyltriethoxysilane, n-hexadecyltriethoxysilane,n-octadecyltriethoxysilane, methyl n-octyldiethoxysilane, and the like.In a more suitable embodiment, the alkyl alkoxysilane comprisesn-octyltriethoxysilane.

Although alkyl alkoxysilanes employing methoxysilane groups can be used,it is preferred for environmental reasons that ethoxysilanes areemployed, rather than methoxysilanes, because ethyl alcohol, rather thanmethyl alcohol, will be released when the alkoxysilane portion of thecoupling agent reacts with the surface of the silica particle.

The elastomers containing unsaturated carbon-carbon bonds in themolecular structure are those that are typically employed withinvulcanizable compositions that are useful for making tires and tirecomponents and include both natural and synthetic elastomers. As usedherein, the term elastomer or rubber will refer to a blend of syntheticand natural rubber, a blend of various synthetic rubbers, or simply onetype of elastomer or rubber. When the preferred polymers are blendedwith conventional rubbers, the amounts can vary widely within a rangecomprising from about one to about 100 percent by weight of the totalrubber, with the conventional rubber or rubbers making up the balance ofthe total rubber (100 parts).

As discussed further below, the elastomer is preferably selected fromthe group consisting of homopolymers of conjugated diene monomers, andcopolymers and terpolymers of the conjugated diene monomers withmonovinyl aromatic monomers and trienes. For example, suitableelastomers include, but are not limited to, natural rubber,polyisoprene, styrene/butadiene rubber, polybutadiene, butadienecopolymers and terpolymers comprising greater than about 35% butadieneby weight, butyl rubber, neoprene, ethylene/propylene diene rubber,acrylonitrile/butadiene rubber (NBR), and mixtures and blends thereof.In many cases butadiene/isoprene copolymer, butadiene/isoprene/styreneterpolymer, and styrene/butadiene copolymer can be used.

The diene elastomers, or copolymers or terpolymers of conjugated dienemonomers and monovinyl aromatic monomers, can be utilized as 100 partsof the rubber in the treadstock compound, or they can be blended withany conventionally employed treadstock rubber which includes naturalrubber, synthetic rubber and blends thereof. Such rubbers are well knownto those skilled in the art and include synthetic polyisoprene rubber,styrene-butadiene rubber (SBR), styrene-isoprene-butadiene rubber,styrene-isoprene rubber, butadiene-isoprene rubber, polybutadiene, butylrubber, neoprene, ethylene-propylene rubber, ethylene-propylene-dienerubber (EPDM), acrylonitrile-butadiene rubber (NBR), silicone rubber,the fluoroelastomers, ethylene acrylic rubber, ethylene vinyl acetatecopolymer (EVA), epichlorohydrin rubbers, chlorinated polyethylenerubbers, chlorosulfonated polyethylene rubbers, hydrogenated nitrilerubber, tetrafluoroethylene-propylene rubber and the like. When thevulcanizable elastomeric composition of the present invention is blendedwith conventional rubbers, the amounts can vary widely with a lowerlimit comprising about 10 percent to 20 percent by weight of the totalrubber. The minimum amount will depend primarily upon the physicalproperties desired.

The vulcanizable rubber compositions according to the invention cancomprise any solution polymerizable or emulsion polymerizable elastomer.Solution and emulsion polymerization techniques are well known to thoseof ordinary skill in the art. The elastomers that are useful inpracticing this invention can also include any of the variousfunctionalized polymers that are conventionally employed in the art ofmaking tires. For example, polymers can be terminally functionalized, orfunctionalized throughout the polymer backbone, such as with functionalgroups derived from an anionic polymerization initiator or a terminatingor coupling agent. Preparation of functionalized polymers is well knownto those skilled in the art. Exemplary methods and agents forfunctionalization of polymers are disclosed, for example, in U.S. Pat.Nos. 5,268,439, 5,496,940, 5,521,309 and 5,066,729, the disclosures ofwhich are hereby incorporated by reference. For example, compounds thatprovide terminal functionality that are reactive with the polymer boundcarbon-lithium moiety can be selected to provide a desired functionalgroup. Examples of such compounds are alcohols, substituted aldimines,substituted ketimines, Michler's ketone, 1,3-dimethyl-2-imidazolidinone,1-alkyl substituted pyrrolidinones, 1-aryl substituted pyrrolidonones,tin tetrachloride, tributyl tin chloride, carbon dioxide, and mixturesthereof. Other useful terminating agents can include those of thestructural formula (R)_(a)ZX_(b), where Z is tin or silicon, R is analkyl having from about one to about 20 carbon atoms, a cycloalkylhaving from about 3 to about 30 carbon atoms; and aryl having from about6 to about 20 carbon atoms, or an aralkyl having from about 7 to about20 carbon atoms. For example, R can include methyl, ethyl, n-butyl,neophyl, phenyl, cyclohexyl, or the like. X is a halogen, such aschlorine or bromine, or alkoxy (—OR), “a” is an integer from zero to 3,and “b” is an integer from one to 4, where a+b=4. Examples of suchterminating agents include tin tetrachloride, tributyl tin chloride,butyl tin trichloride, butyl silicon trichloride, as well astetraethoxysilane, Si(OEt)₄, and methyl triphenoxysilane, MeSi(OPh)₃.The practice of the present invention is not limited solely to polymersterminated with these agents, since other compounds that are reactivewith the polymer bound carbon-lithium moiety can be selected to providea desired functional group.

The elastomeric compound can be sulfur vulcanized and prepared by thesteps of (a) mixing together at a temperature of about 130° C. to about200° C. in the absence of added sulfur and cure agents, an elastomeroptionally having an alkoxysilane terminal group and containingunsaturated carbon-carbon bonds in its molecular structure, areinforcing inorganic filler, carbon black, or a mixture of theinorganic filler with carbon black, an alkyl alkoxysilane, and anon-sulfur coupling agent, as described above, wherein the ratio of thecoupling agent to the alkyl alkoxysilane is about 0.0001:1 to about 1:1;(b) allowing the mixture to cool below the mixing temperature; (c)mixing the mixture obtained in step (b), at a temperature lower than avulcanization temperature, with a cure agent and an effective amount ofsulfur to achieve a satisfactory cure; and (d) curing the mixtureobtained in step (c). The compound is usually cured at about 140° C. toabout 190° C. for about 5 to about 120 minutes.

Depending on the compound desired, the initial step of the method oftenrequires that the mixture reaches a temperature from about 130° C. toabout 200° C., about 155° C. to about 200° C., about 165° C. to about195° C., more often about 170° C. to about 190° C., especially about170° C. to about 185° C. In one arrangement of the invention, theinitial mixing step can include at least two substeps. That is, theinitial mixing step can comprise a first substep (i) mixing together theelastomer, at least a portion of the inorganic reinforcing filler, atleast a portion of alkyl alkoxysilane and at least a portion of thecoupling agent, with an optional intervening cooling step; and a secondsubstep (ii) mixing the mixture obtained in step (i) with the remainderof the inorganic reinforcing filler, if any, and the remainder of thealkyl alkoxysilane and/or coupling agent if any. The method can furtherinclude a remill step in which either no ingredients are added to thefirst mixture, or non-curing ingredients are added, in order to reducethe compound viscosity and improve the dispersion of the inorganicreinforcing filler. The temperature of the remill step is typicallyabout 130° C. to about 175° C., especially about 145° C. to about 165°C.

The final step of the mixing process is the addition of cure agents tothe mixture, including an effective amount of sulfur to achieve asatisfactory cure of the final compound. The temperature at which thefinal mixture is mixed must be below the vulcanization temperature inorder to avoid unwanted precure of the compound. Therefore, thetemperature of the final mixing step should not exceed about 120° C. andis typically about 40° C. to about 115° C., about 60° C. to about 110°C. and, especially, about 75° C. to about 100° C.

The order of addition of the reinforcing filler, alkyl alkoxysilane andcoupling agent to the mixer in the initial step of the method is notcritical. The alkyl alkoxysilane and/or the mercaptosilane can be addedprior to or after the addition of the filler. In one arrangement of themethod, a portion of the reinforcing filler and the coupling agentand/or the alkyl alkoxysilane are added simultaneously to the mixer. Forexample, the coupling agent and/or the alkyl alkoxysilane can bepartially or fully supported on the inorganic reinforcing filler and/orthe carbon black reinforcing filler. The ratio of the amount ofsupported silane to the filler is not critical. If the silane is aliquid, a suitable ratio of supported silane to filler is that whichresults in a suitably dry material for addition to the elastomer. Forexample, the ratio can be about 1/99 to about 70/30, about 20/80, about60/40, about 50/50, and the like.

Based on the disclosure contained herein, and in the examples ofinvention compositions described below, one skilled in the art of rubbercompounding can easily determine the effective amount of sulfur requiredfor a satisfactory cure of the compound without undue experimentation.The additional sulfur can take any form, including soluble sulfur,insoluble sulfur, or any of the sulfur-donating compounds described asvulcanizing agents below, or mixtures of the foregoing.

The vulcanizable rubber compositions of the invention are preferablycompounded with inorganic reinforcing fillers, such as silica or carbonblack, or a mixture of silica and carbon black. Examples of suitablesilica reinforcing filler include, but are not limited to, precipitatedamorphous silica, wet silica (hydrated silicic acid), dry silica(anhydrous silicic acid), fumed silica, calcium silicate, and the like.Other suitable fillers include aluminum silicate, magnesium silicate,and the like. Among these, precipitated amorphous wet-process, hydratedsilicas are preferred. These silicas are so-called because they areproduced by a chemical reaction in water, from which they areprecipitated as ultrafine, spherical particles. These primary particlesstrongly associate into aggregates, which in turn combine less stronglyinto agglomerates. The surface area, as measured by the BET method givesthe best measure of the reinforcing character of different silicas. Forsilicas of interest for the present invention, the surface area shouldbe about 32 m²/g to about 400 m²/g, with the range of about 100 m²/g toabout 250 m²/g being preferred, and the range of about 150 m²/g to about220 m²/g being most preferred. The pH of the silica filler is generallyabout 5.5 to about 7 or slightly over, preferably about 5.5 to about6.8.

Silica can be employed in the amount of about one to about 150 parts byweight per hundred parts of the elastomer (phr), preferably in an amountof about five to about 80 phr and, more preferably, in an amount ofabout 30 to about 80 phr. The useful upper range is limited by the highviscosity imparted by fillers of this type. Some of the commerciallyavailable silicas which can be used include, but are not limited to,Hi-Sil® 190, Hi-Sil® 210, Hi-Sil® 215, Hi-Sil® 233, Hi-Sil® 243, and thelike, produced by PPG Industries (Pittsburgh, Pa.). A number of usefulcommercial grades of different silicas are also available from DegussaCorporation (e.g., VN2, VN3), Rhone Poulenc (e.g., Zeosil® 1165 MP), andJ. M. Huber Corporation.

The elastomers can be compounded with all forms of carbon black alone,or in a mixture with the silica. The carbon black can be present inamounts ranging from about one to about 90 phr, with about five to about60 phr being preferred. The carbon blacks can include any of thecommonly available, commercially-produced carbon blacks, but thosehaving a surface area (EMSA) of at least 20 m²/g and, more preferably,at least 35 m²/g up to 200 m²/g or higher are preferred. Surface areavalues used in this application are determined by ASTM D-1765 using thecetyltrimethyl-ammonium bromide (CTAB) technique. Among the usefulcarbon blacks are furnace black, channel blacks and lamp blacks. Morespecifically, examples of useful carbon blacks include super abrasionfurnace (SAF) blacks, high abrasion furnace (HAF) blacks, fast extrusionfurnace (FEF) blacks, fine furnace (FF) blacks, intermediate superabrasion furnace (ISAF) blacks, semi-reinforcing furnace (SRF) blacks,medium processing channel blacks, hard processing channel blacks andconducting channel blacks. Other carbon blacks which can be utilizedinclude acetylene blacks. A mixture of two or more of the above blackscan be used in preparing the carbon black products of the invention.Typical suitable carbon blacks are N-110, N-220, N-339, N-330, N-351,N-550 and N-660, as designated by ASTM D-1765-82a. The carbon blacksutilized in the preparation of the vulcanizable elastomeric compositionsof the invention can be in pelletized form or an unpelletized flocculentmass. Preferably, for more uniform mixing, unpelletized carbon black ispreferred.

The vulcanizable rubber compositions of the invention can optionallyfurther include an additional silica coupling agent such as, but notlimited to, mercaptosilane(s), blocked mercaptosilane(s),bis(trialkoxysilylorgano)polysulfide(s), 3-thiocyanatopropyltrimethoxysilane, silanes that are carried on a filler such as silica,carbon black and the like, or any of the silica coupling agents that areknown to those of ordinary skill in the rubber compounding art.Exemplary mercaptosilanes include, but are not limited to,1-mercaptomethyltriethoxysilane, 2-mercaptoethyltriethoxysilane,3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldiethoxysilane,2-mercaptoethyltriproxysilane, 18-mercaptooctadecyldiethoxychlorosilane,and the like. The mercaptosilane can be present in the compound in anamount of about 0.0001% to about 3% by weight, typically about 0.001% toabout 1.5% by weight, and especially about 0.01% to about 1% by weight,based on the weight of the silica. Exemplary blocked mercaptosilanesinclude, but are not limited to, octanoyl blocked3-mercaptopropyltriethoxysilane, and the like.

Exemplary bis(trialkoxysilylorgano)polysulfide silica coupling agentsinclude, but are not limited to,bis(3-triethoxysilyl-propyl)tetrasulfide (TESPT), which is soldcommercially under the tradename Si69 by Degussa Inc., New York, N.Y.,and bis(3-triethoxysilylpropyl) disulfide (TESPD) or Si75, availablefrom Degussa, or Silquest® A1589, available from Crompton. Thepolysulfide organosilane silica coupling agent can be present in anamount of about 0.01% to about 20% by weight, based on the weight of thesilica, preferably about 0.1% to about 15% by weight, and especiallyabout 1% to about 10%.

Exemplary additional silica shielding agents suitable for use in theinvention include, but are not limited to an alkyl alkoxysilane, analkoxy-modified silsesquioxane (AMS) an mercaptan/alkoxy-modifiedco-AMS, an amino AMS, an amino/mercaptan co-AMS, a fatty acid ester of ahydrogenated or non-hydrogenated C₅ or C₆ sugar, a polyoxyethylenederivative of a fatty acid ester of a hydrogenated or non-hydrogenatedC₅ or C₆ sugar, and mixtures thereof or a mineral or non-mineraladditional filler, as described in greater detail below. Exemplary fattyacid esters of hydrogenated and non-hydrogenated C₅ and C₆ sugars (e.g.,sorbose, mannose, and arabinose) that are useful as silica dispersingaids include, but are not limited to, the sorbitan oleates, such assorbitan monooleate, dioleate, trioleate and sesquioleate, as well assorbitan esters of laurate, palmitate and stearate fatty acids. Fattyacid esters of hydrogenated and non-hydrogenated C₅ and C₆ sugars arecommercially available from ICI Specialty Chemicals (Wilmington, Del.)under the trade name SPAN®. Representative products include SPAN® 60(sorbitan stearate), SPAN® 80 (sorbitan oleate), and SPAN® 85 (sorbitantrioleate). Other commercially available fatty acid esters of sorbitanare also available, such as the sorbitan monooleates known as Alkamul®SMO; Capmul® O; Glycomul® O; Arlacel® 80; Emsorb® 2500; and S-Maz® 80. Auseful amount of these additional silica dispersing aids when used withthe bis(trialkoxysilylorgano) polysulfide silica coupling agents isabout 0.1% to about 25% by weight based on the weight of the silica,with about 0.5% to about 20% by weight being preferred, and about 1% toabout 15% by weight based on the weight of the silica being morepreferred. In the alkyl alkoxysilane and mercaptosilane embodiment ofthe invention, it may be desirable to use about 0.1% to about 20% byweight of the fatty acid ester based on the weight of the silica. Estersof polyols, including glycols such as polyhydroxy compounds and thelike, polyethylene oxides, polyethers, and the like, in the samequantities, are also useful in all invention embodiments.

Exemplary polyoxyethylene derivatives of fatty acid esters ofhydrogenated and non-hydrogenated C₅ and C₆ sugars include, but are notlimited to, polysorbates and polyoxyethylene sorbitan esters, which areanalogous to the fatty acid esters of hydrogenated and non-hydrogenatedsugars noted above except that ethylene oxide groups are placed on eachof the hydroxyl groups. Representative examples of polyoxyethylenederivatives of sorbitan include POE® (20) sorbitan monooleate,Polysorbate® 80, Tween® 80, Emsorb® 6900, Liposorb® 0-20, T-Maz® 80, andthe like. The Tween® products are commercially available from ICISpecialty Chemicals. Generally, a useful amount of these optional silicadispersing aids is about 0.1% to about 25% by weight based on the weightof the silica, with about 0.5% to about 20% by weight being preferred,and about 1% to about 15% by weight based on the weight of the silicabeing more preferred.

The silica coupling agents, silica shielding agents and/or other silicadispersing aids and/or other liquid components of the composition can befully or partially supported by the reinforcing filler. The ratio of thecomponent to the reinforcing filler is not critical. If the dispersingaid is a liquid, a suitable ratio of dispersing aid to filler is thatwhich results in a suitably dry material for addition to the elastomer.For example, the ratio can be about 1/99 to about 70/30, about 20/80about 60/40, about 50/50, and the like.

Certain additional fillers can be utilized according to the presentinvention as processing aids, including mineral fillers, such as clay(hydrous aluminum silicate), talc (hydrous magnesium silicate), aluminumhydrate [Al(OH)₃] and mica, as well as non-mineral fillers such as ureaand sodium sulfate. Preferred micas principally contain alumina andsilica, although other known variants are also useful. The foregoingadditional fillers are optional and can be utilized in the amount ofabout 0.5 to about 40 phr, preferably in an amount of about one to about20 phr and, more preferably in an amount of about one to about 10 phr.These additional fillers can also be used as non-reinforcing fillers tosupport the strong organic base catalysts, as well as any of the silicadispersing aids, and silica coupling agents described above. As with thesupport of the silica dispersing aid on the reinforcing filler, asdescribed above, the ratio of dispersing aid to non-reinforcing filleris not critical. For example, the ratio can be about 1/99 to about70/30, about 20/80, about 60/40, about 50/50, and the like, by weight.

The vulcanizable rubber compositions are compounded or blended by usingmixing equipment and procedures conventionally employed in the art, suchas mixing the various vulcanizable polymer(s) with reinforcing fillersand commonly used additive materials such as, but not limited to, curingagents, activators, retarders and accelerators; processing additives,such as oils; resins, including tackifying resins; plasticizers;pigments; additional fillers; fatty acid; zinc oxide; waxes;antioxidants; antiozonants; peptizing agents; and the like. As known tothose skilled in the art, the additives mentioned above are selected andcommonly used in conventional amounts.

The vulcanizable rubber composition can then be processed according toordinary tire manufacturing techniques Likewise, the tires areultimately fabricated by using standard rubber curing techniques. Forfurther explanation of rubber compounding and the additivesconventionally employed, one can refer to The Compounding andVulcanization of Rubber, by Stevens in Rubber Technology, Second Edition(1973 Van Nostrand Reibold Company), which is incorporated herein byreference. The reinforced rubber compounds can be cured in aconventional manner with known vulcanizing agents at about 0.1 to 10phr. For a general disclosure of suitable vulcanizing agents, one canrefer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed.,Wiley Interscience, N.Y. 1982, Vol. 20, pp. 365 to 468, particularlyVulcanization Agents and Auxiliary Materials, pp. 390 to 402, orVulcanization by A. Y. Coran, Encyclopedia of Polymer Science andEngineering, Second Edition (1989 John Wiley & Sons, Inc.), both ofwhich are incorporated herein by reference. Vulcanizing agents can beused alone or in combination. Preferably, the rubber compounds aresulfur-vulcanized. Cured or crosslinked polymers will be referred to asvulcanizates for purposes of this disclosure.

Such elastomeric compositions, when vulcanized using conventional rubbervulcanization conditions, exhibit reduced hysteresis, which means aproduct having increased rebound, decreased rolling resistance andlessened heat build-up when subjected to mechanical stress. Productsincluding tires, power belts and the like are envisioned. Decreasedrolling resistance is, of course, a useful property for pneumatic tires,both radial as well as bias ply types and thus, the vulcanizableelastomeric compositions of the present invention can be utilized toform treadstocks for such tires. Pneumatic tires can be made accordingto the constructions disclosed in U.S. Pat. Nos. 5,866,171; 5,876,527;5,931,211; and 5,971,046, the disclosures of which are incorporatedherein by reference. The composition can also be used to form otherelastomeric tire components such subtreads, sidewalls, body ply skims,bead fillers, apex, chafer, sidewall insert, wirecoat, inner liner, andthe like.

EXAMPLES

The following examples are prophetic and illustrate methods ofpreparation of representative non-sulfur coupling agents, in combinationwith an alkyl alkoxysilane (n-octyltriethoxysilane), inorganicreinforcing fillers (silica in combination with carbon black), andrubber compounds and tire components containing them. However, theexamples are not intended to be limiting, as other similar couplingagents can be prepared according to the described methods and otherratios of the coupling agent to the alkyl alkoxysilane can be used, aswell as other inorganic reinforcing fillers. Moreover, the methods areexemplary only and other methods for preparing the coupling agents andother rubber compounds, including different compounding formulations andreinforcing fillers, can be determined by those skilled in the artwithout departing from the scope of the invention herein disclosed andclaimed.

Example 1 Preparation of a Maleimide Triethoxysilane Coupling Agent

N-(3-propyl triethoxysilyl) maleimide (PSM) is prepared by the additionof 100 mL of a 2.33 molar saturated anhydrous toluene solution of maleicanhydride (22.85 g of anhydride, 0.233 mol) to a one liter flaskcontaining 100 mL of dry toluene and then, under nitrogen, slowlydripping in a solution of 3-aminopropyl triethoxysilane (51.6 g, 54.8mL, 0.233 mol) dissolved in 100 mL of dry toluene over a 15 minuteperiod. After stirring 2 hours, anhydrous zinc chloride (10 g, 73.3mmol) and hexamethyldisilazane (HMDS) (47.0 g, 60.7 mL, 0.291 mol)dissolved in 100 mL of dry toluene are added. The solution is heated to80° C. for 18 hrs with stifling before being filtered and isolated bydistillation.

Example 2 Preparation of Azo-Bis-Carbonyl Triethoxysilane CouplingAgents

An N-(3-propyl triethoxysilyl) (NPTS) group can be added to a variety ofazo-bis-carbonyl compounds to prepare eneophiles that will readily reactwith double bonds in a diene rubber.

(A) Reaction of Ethyl Carbazate (EC) with Isocyanates

To a solution of ethyl carbazate (20.8 g, 0.2 mol) in 200 mL of drytetrahydrofuran (THF) is added a solution of 3-isocyanatopropyltriethoxysilane (49.5 g, 50 mL, 0.2 mol, available from Gelest) in 50 mLof dry THF. The solvent is removed by heating with vacuum or a nitrogenpurge to give an intermediate that is then suspended in 500 mL of drymethylene chloride with anhydrous sodium sulfate (35.5 g, 0.25 mol)added as a drying agent. Into this slurry is slowly bubbled dinitrogentetroxide until brown vapor of the unreacted gas persists above thesolution. Filtration and removal of the solvent allows the isolation ofthe solid product coupling agent.

(B) Reaction of Ethyl Carbazate (EC) with Chloroformates

The chloroformate of the 4-hydroxybutyramide of NPTS is prepared by thediluting 100 mL of an approximate 20% toluene solution of phosgene (19.8g, 0.2 mol) to a total of 200 mL with more dry toluene. This solution isthen added to a solution of N-(3-triethoxysilylpropyl)-4-hydroxybutyramide (61.5 g, 60.3 mL, 0.2 mol, Gelest) anddimethyl aniline (24.2 g, 0.2 mol) dissolved in 200 mL of THF. Afterheating 3 hrs at 80° C. the solution is cooled, filtered free of thedimethyl aniline hydrochloride and concentrated. The chloroformateobtained is reacted in 200 mL of THF containing the EC (20.8 g, 0.2 mol)and triethylamine (20.2 g, 27.9 mL, 0.2 mol) by heating for 3 hrs atreflux, cooling and filtering. Then, oxidization in methylene chlorideas described above in (A) gives the desired coupling agent.

(C) Reaction of Ethyl Carbazate (EC) with Acid Chlorides

The acid chloride of triethoxy propyl maleamic acid (63.9 g, 0.2 mol)(available from Gelest) was prepared in 300 mL of THF by addition ofthionyl chloride (23.8 g, 14.5 mL, 0.2 mol) and heating to reflux for 1hrs. The solution was cooled, reacted with EC as above in (B) to obtainthe desired coupling agent.

Example 3 Preparation of a Nitrone Coupling Agents By 1,3 DipolarAddition

A solution of triethoxysilyl undecanal (66.5 g, 0.2 mol, available fromGelest) in 250 mL of absolute ethanol is added to phenyl hydroxylaminehydrochloride (29.1 g, 0.2 mol) and triethylamine (30.3 g, 41.9 mL, 0.3mol) and allowed to stand at ambient temperature for 20 hrs. The ethanoland excess amine are removed by heat and vacuum. The desired nitroneproduct is readily separated from the residue by washing it free of theamine hydrochloride with methylene chloride.

Example 4 Evaluation of the Prepared Coupling Agents in a Diene RubberComposition

The silica/rubber coupling agents prepared in Examples 1 through 3 areevaluated in a rubber composition illustrated in Table 1. The rubberstocks are prepared with various amounts of the coupling agents and thealkyl alkoxysilane, and the control stock is prepared with the3-mercaptopropyltriethoxysilane and the alkyl alkoxysilane. The controlstock was described in our U.S. Pat. No. 6,433,065, and was shown toprovide desirable processability and improved tensile mechanical anddynamic viscoelastic properties. The silica/rubber coupling agent isadded in the master batch stage; although it could be added in theremill stage or any other stage prior to the final stage. All of thecharges are listed as parts per hundred rubber (phr). The droptemperatures of the mixing steps are as follows: master batch, 154° C.;remill, 143° C.; and final, 105° C. All of the compounded final stocksare sheeted and subsequently cured at 171° C. for 15 minutes.

Example 5 A. Processing Evaluation of Rubber Compound

The processing of the green stocks (i.e., the stock obtained after thefinal stage, prior to curing) is characterized as to Mooney viscosityand cure characteristics. The Mooney viscosity measurement is conductedat 130° C. using a large rotor. The Mooney viscosity is recorded as thetorque when the rotor is rotated for 4 minutes. The samples arepreheated at 130° C. for one minute before the rotor is started. AMonsanto Rheometer MD2000 is used to characterize the stock curingprocess. The frequency is 1.67 Hz and the strain is 7% at 160° C. TheT_(s2) and T₉₀ are obtained from these measurements and represent thetime when the torque rises to 2% and 90%, respectively, of the totaltorque increase during the curing process. These measurements are usedto predict how quickly the viscosity built up (Ts₂) and the curing rate(T90) during the curing process.

The ML₄ of the compounded stock at 130° C. increases as the amount ofalkyl alkoxysilane increases with respect to the amount of the couplingagent in the composition. That is, the viscosity decreases as the ratioof coupling agent to alkyl alkoxysilane decreases.

B. Tire Performance Predicted Based on the Measured Dynamic ViscoelasticMechanical Properties

The dynamic viscoelastic properties for the stocks are obtained fromtemperature sweep tests conducted with a frequency of 31.4 rad/sec using0.5% strain for temperatures ranging from −100° C. to −20° C., and 2%strain for temperatures ranging from −20° C. to 100° C. The stockscontaining the indicated ratio of coupling agent and alkyl alkoxysilanehave a value of 50° C. tan δ, and stress and strain properties similarto the control stock containing a mercaptosilane and alkyl alkoxysilanein a similar ratio.

These favorable properties of the non-sulfur coupling agent and alkylalkoxysilane combination-containing stock are confirmed by measuring thedynamic viscoelastic properties using the dynamic compression test andrebound tests. The sample geometry used for the dynamic compression testis a cylindrical button having a diameter of 9.5 mm and a length of 15.6mm. The sample is compressed under a static load of 2 kg before testing.After an equilibrium state is reached, the test was started with adynamic compression load of 1.25 kg at a frequency of 1 Hz. The sampleis dynamically compressed and then extended, and the resultantdisplacement K′, the dynamic storage compression modulus K″ and thehysteresis (tan δ) were then recorded, as well as the resilience of therubber compound (rebound test).

The Zwick rebound resilience tester measures rebound resilience as avery basic dynamic test. The test piece is subjected to one-half cycleof deformation. The sample geometry is round with a diameter of 38.1 mmand a thickness of 1.9 mm. The specimen is strained by impacting thetest piece with an indentor which is free to rebound after the impact.The rebound resilience is defined as the ratio of mechanical energiesbefore and after impact. Samples are preheated 30 minutes prior totesting.

TABLE 1 Rubber Formulation Ingredient Amount (phr) Styrene-butadienerubber* 100 Carbon black 35 Silica 30 Silica/Rubber Coupling Agent**Varied Alkyl alkoxysilane (OTES)^(†) Varied3-mercaptopropyltriethoxysilane Varied Antioxidant - 6PPD 0.95Naphthenic oil 12.5 Wax 1.0 Zinc oxide 2.5 Sulfur 2 CBS^(††) 1.5DPG^(††) 0.8 *SBR -- 23.5% styrene, T_(g) = −36° C., ML₄ = 58 **Aninvention coupling agent prepared as in examples 1-3 or a control 3-mercaptopropyl triethoxysilane carried on silica (Ciptane ® 255LD fromPPG Industries). ^(†)OTES = n-octyltriethoxysilane ^(††)CBS =N-cyclohexyl-2-benzothiazolesulfenamide; DPG = diphenyl guanidine

While the invention has been described herein with reference to thepreferred embodiments, it is to be understood that it is not intended tolimit the invention to the specific forms disclosed. On the contrary, itis intended that the invention cover all modifications and alternativeforms falling within the scope of the appended claims.

1. A vulcanizable rubber composition comprising: (a) an elastomercontaining unsaturated carbon-carbon bonds in its molecular structure;(b) a reinforcing inorganic filler, carbon black, or a mixture of theinorganic filler with carbon black; (c) a coupling agent comprising analkoxysilane moiety that binds to the inorganic filler and a non-sulfurmoiety that reacts with the unsaturated carbon-carbon bonds of theelastomer to bind the coupling agent to the elastomer; (d) an alkylalkoxysilane, wherein the weight ratio of the coupling agent to thealkyl alkoxysilane is about 0.0001:1 to about 1:1; and (e) a cure agent;wherein the non-sulfur moiety that reacts with the unsaturatedcarbon-carbon bonds of the elastomer to bind the coupling agent to theelastomer is: (i) —N(C═O)₂N═N, (ii) —N(C═O)₂CH═CR¹, (iii)—NH(C═O)N═N(C═O)OR¹, (iv) —(C═O)N═N(C═O)OR¹; and (v) —O(C═O)N═N(C═O)OR¹;and mixtures thereof; wherein R¹ is C₁ to C₄ alkyl.
 2. The rubbercomposition of claim 1, wherein the weight ratio of the coupling agentto the alkyl alkoxysilane is about 0.001:1 to about 0.80:1.
 3. Therubber composition of claim 1, wherein the coupling agent is present inan amount of about 0.0001% to about 3% by weight, based on the weight ofthe silica.
 4. The rubber composition of claim 3, wherein the alkylalkoxysilane is present in an amount of about 0.1% to about 20% byweight, based on the silica.
 5. The rubber composition of claim 1,wherein the non-sulfur moiety of the coupling agent consists essentiallyof an eneophile.
 6. The rubber composition of claim 5, wherein thecoupling agent comprises an imidoalkoxysilane, anazo-bis-carbonyl-alkoxysilane, or mixtures thereof.
 7. The rubbercomposition of claim 1, wherein the inorganic filler is selected fromthe group consisting of silicates, talc, kaolin, clay, metal oxides,aluminum hydrate, mica, and mixtures thereof.
 8. The rubber compositionof claim 1, wherein the elastomer is selected from the group consistingof homopolymers of a conjugated diene monomer, and copolymers andterpolymers of the conjugated diene monomers with monovinyl aromaticmonomers and trienes.
 9. The rubber composition of claim 8, wherein theelastomer is selected from the group consisting of natural rubber,polyisoprene, styrene/butadiene rubber, polybutadiene, butadienecopolymers and terpolymers comprising greater than about 35% butadieneby weight, butyl rubber, neoprene, ethylene/propylene diene rubber,acrylonitrile/butadiene rubber, and mixtures and blends thereof.
 10. Therubber composition of claim 9, wherein the elastomer is selected fromthe group consisting of butadiene/isoprene copolymer,butadiene/isoprene/styrene terpolymer, and styrene/butadiene copolymer.11. The rubber composition of claim 1, wherein the alkyl alkoxysilanecompound has the formula R¹ _(p)Si(OR²)₄-p, wherein the alkoxy groupsare the same or different from each other, each R¹ independentlycomprises C₁, to about C₂₀ aliphatic, about C₅ to about C₂₀cycloaliphatic, or about C₆ to about C₂₀ aromatic, each R² independentlycomprises C₁ to about C₆ aliphatic, and p is an integer from 1 to
 3. 12.The rubber composition of claim 11, wherein the alkyl alkoxysilane isselected from the group consisting of octyltriethoxysilane,octyltrimethoxysilane, trimethylethoxysilane, cyclohexyltriethoxysilane,isobutyltriethoxysilane, ethyltrimethoxysilane,cyclohexyl-tributoxysilane, dimethyldiethoxysilane,methyltriethoxysilane, propyltriethoxysilane, hexyltriethoxysilane,heptyltriethoxysilane, nonyltriethoxysilane, decyltriethoxy-silane,dodecyltriethoxysilane, tetradecyltriethoxysilane,octadecyltri-ethoxysilane, methyloctyl-diethoxysilane,dimethyldimethoxysilane, methyltri-methoxysilane,propyltrimethoxy-silane, hexyltrimethoxysilane, heptyltrimethoxy-silane,nonyltrimethoxysilane, decyltrimethoxysilane, dodecyltrimethoxysilane,tetradecyltrimethoxysilane, octadecyltrimethoxysilane, methyloctyldimethoxysilane, and mixtures thereof.
 13. The rubber composition ofclaim 12, wherein the alkyl alkoxysilane comprisesn-octyltriethoxysilane.
 14. The rubber composition of claim 1, furthercomprising an additional shielding agent for the inorganic filler. 15.The rubber composition of claim 14 wherein the additional shieldingagent is selected from the group consisting of an additional alkylalkoxysilane, an alkoxy-modified silsesquioxane, a fatty acid ester of ahydrogenated or non-hydrogenated C₅ or C₆ sugar, a polyoxyethylenederivative of a fatty acid ester of a hydrogenated or non-hydrogenatedC₅ or C₆ sugar, an ester of a polyol, a polyethylene oxide, a polyether,and mixtures thereof.
 16. The rubber composition of claim 15, whereinthe fatty acid ester is selected from the group consisting of sorbitanmonooleate, sorbitan dioleate, sorbitan trioleate, sorbitansesquioleate, sorbitan laurate, sorbitan palmitate, sorbitan stearate,and mixtures thereof.
 17. A vulcanized rubber compound comprising thevulcanizable rubber composition of claim
 1. 18. A pneumatic tirecomprising a component produced from a vulcanized rubber compositioncomprising an elastomer containing unsaturated carbon-carbon bonds inits molecular structure; a reinforcing inorganic filler, carbon black,or a mixture of the inorganic filler with carbon black; a coupling agentcomprising an alkoxysilane moiety that binds to the inorganic filler anda non-sulfur moiety that reacts with the unsaturated carbon-carbon bondsof the elastomer to bind the coupling agent to the elastomer; an alkylalkoxysilane, wherein the weight ratio of the coupling agent to thealkyl alkoxysilane is about 0.0001:1 to about 1:1; and a cure agent;wherein the non-sulfur moiety that reacts with the unsaturatedcarbon-carbon bonds of the elastomer to bind the coupling agent to theelastomer is: (i) —N(C═O)₂N═N, (ii) —N(C═O)₂CH═CR¹, (iii)—NH(C═O)N═N(C═O)OR¹, (iv) —(C═O)N═N(C═O)OR¹; (v) —O(C═O)N═N(C═O)OR¹; andmixtures thereof; wherein R¹ is C₁ to C₄ alkyl.
 19. The pneumatic tireof claim 18, wherein the component is selected from the group consistingof treads, subtreads, sidewalls, body ply skims, bead fillers, apex,chafer, sidewall insert, wirecoat, inner liner, and combinationsthereof.
 20. A vulcanizable rubber composition comprising: (a) anelastomer containing unsaturated carbon-carbon bonds in its molecularstructure; (b) a reinforcing inorganic filler, carbon black, or amixture of the inorganic filler with carbon black; (c) a coupling agentcomprising an alkoxysilane moiety that binds to the inorganic filler anda non-sulfur moiety that reacts with the unsaturated carbon-carbon bondsof the elastomer to bind the coupling agent to the elastomer; (d) analkyl alkoxysilane, wherein the weight ratio of the coupling agent tothe alkyl alkoxysilane is about 0.20:1 to about 1:1; and (e) a cureagent.
 21. The rubber composition of claim 1, wherein the coupling agentcomprises the formula:Y—(R³)(R²)_((3-a))Si(OR¹)_(a), wherein R¹ is C₁ to C₄ alkyl; R² is C₁ toC₂₀ alkyl, C₃ to C₂₀ cycloalkyl, C₆ to C₂₀ aromatic or C₅ to C₂₀heteroaromatic; R³ is C₁ to C₂₀ alkylenyl, C₃ to C₂₀ cycloalkylenyl, C₆to C₂₀ arylenyl, C₅ to C₂₀ heteroarylenyl, or R⁴—CH═CH—R⁴, where R⁴ is asingle bond or an R³; a=1, 2 or 3; and Y is the non-sulfur moiety.